Camera head for fast reproduction of a vaccum uv spectrometer spectrum

ABSTRACT

The invention relates to an apparatus comprising a spectrometer, means for the spatially-resolving conversion into charge of light exiting the spectrometer, an integrator circuit for the spatially-resolving integration of the charge, as well as means for displaying the charge that is integrated dependent on the position. The integrator circuit is assembled from discrete components. Thus, the transport of plasma impurities can be properly measured.

FIELD OF THE INVENTION

The invention relates to an apparatus for measuring the transport timeof impurity particles in a plasma, comprising a spectrometer and aline-scan camera. The spectrometer has an entrance slit. The spectrallines emitted by the impurity particles fall through the entrance slit.Then, the spectral lines fall onto a diffraction grating. Depending onwavelength, the spectral lines are reflected at different angles. Thesplit light signals leaving the spectrometer are collected by aspatially-resolving line-scan camera. The transport time is calculatedfrom the measurement information thus obtained. During operation, thespectrometer is located in a vacuum atmosphere.

BACKGROUND OF THE INVENTION

For the experimental examination of the radial transport of plasmaimpurities, i.a. a small amount of a plasma impurity is introduced intothe plasma at the plasma edge, and then, the development in time of thespectral lines of different degrees of ionization is measured. In suchexperiments, which are described e.g. in “New diagnostics for physicsstudies on TEXTOR-94”, Review of Scientific Instruments, Vol. 72, Pages1046-1053, the particles that constitute the impurities in theplasma—hereinafter referred to as impurity particles—advance intoregions of increasingly higher electron temperature, starting at theedge of the plasma towards the centre of the plasma, and are graduallyionized to ever higher degrees of ionization. The impurity particles arethen present in the form of ions. At the same time, these ionizedimpurity particles—hereinafter referred to as impurity ions—are excitedto emit characteristic spectral lines. In the experiment, spectral linesof higher degrees of ionization appear delayed in time compared to thoseof lower degrees of ionization. This delay between the appearances oftwo spectral lines of different degrees of ionization, Z₁ and Z₂, is adirect measure for the transport time that the impurity particlesrequire for transport between those radial positions, r₁ and r₂, atwhich the emissivities of the observed spectral lines are located. Theaccuracy of the described method is dependent on the ability to observesimultaneously as many different spectral lines of different degrees ofionization as possible, with a good signal-to-noise ratio and sufficienttime resolution.

Preliminary tests showed for typical conditions in a fusion experimentthat, as a rule, a time resolution of 1 ms is necessary in order to beable to resolve sufficiently the delay between spectral lines ofdifferent degrees of ionization by means of measurement technology.Continuous measurement should be possible over the total dischargeduration of a typical plasma (6 to 10 seconds) in order to be able torecord all processes during the discharge that are interesting tophysics.

The discrimination between spectral lines and the background as well asthe separation of different spectral lines can only be achieved, as arule, by using suitable spectrometers having sufficient wavelengthresolution. The method's applicability to as many different relevantplasma impurities as possible, after all, determines the wavelengthrange in VUV (Vacuum Ultraviolet, approx. 10 nm-100 nm) which is to beobserved, since in this range, the impurities relevant to fusionexperiments show many of their strongest spectral lines.

The requirements with regard to measurement technology and especiallyregarding the spectrometer used for the planned tests can therefore besummarized as follows:

-   -   a) The spectrometer should be able to work over a broad        wavelength range in the VUV (10-100 nm).    -   b) The spectrometer should be able to perform measurements with        a high degree of time resolution, namely at least 1000 complete        spectra per second.    -   c) The efficiency of the spectrometer should be high, since the        signal-to-noise ratio also depends on the photon statistics.    -   d) The spectrometer should have sufficient wavelength resolution        (line separation).    -   e) The spectrometer should have a wide dynamic range since the        intensities of the individual spectral lines vary greatly.

The spectrometer concept “SPRED” (Survey Poor Resolution ExtendedDomain) which is presented in works by Fonck et al., Appl. Optics Vol.21, page 2115 et sqq. (1982), as well as by Stratton et al., Reviews ofScientific instruments Vol. 57, page 2043 et sqq. (1986), was found tobe the closest state of the art at the moment able to meet theabove-mentioned requirements.

The spectrometer concept may be summarized briefly as follows: thecenterpiece of the spectrometer is a diffraction grating by Jobin-Yvonwith the following properties:

-   -   I. Oblique incidence of light at approx. 70 degrees from the        normal of the grating for obtaining sufficient reflectivity of        the coating of the grating in the range of under 50 nm.    -   II. Toroidal grating substrate for reducing the geometric loss        of light through astigmatism. Efficiency is thus enhanced.    -   III. The grooves of the grating are produced by ion etching or        holographically. Thus, a high degree of efficiency is obtained        in the first diffraction order while at the same time, higher        diffraction orders are suppressed. Furthermore, a reduction of        image defects is achieved and a sharp spectrum in a plane of 40        mm width is obtained.    -   IV. The grating surface is coated with gold in order to improve        efficiency in a wavelength range of under 30 nm.    -   V. The lengths of the two arms of the spectrometer (which is to        be understood as being the distance entrance slit—grating or        grating—detector) are chosen to be approx. 30 cm each, so that,        for the size of gratings and grove densities that can be        produced today, the result is an instrument with a large        wavelength range at sufficient wavelength resolution and large        aperture (f/30), and thus with a high degree of efficiency.

The diffraction grating creates images of the entrance slit in the exitplane of the spectrometer of a scale of approx. 1:1. An open MCPdetector (“Multi Channel Plate”) is used in the exit plane of thespectrometer for converting the VUV photons into visible light andamplifying the signals at the same time. The whole spectrometer isoperated in a vacuum because radiation in the wavelength range of 10-100nm is absorbed by all gases and all materials. The operation of the openMCP detectors furthermore necessitates a pressure of under 10⁻⁶ mbar, sothat a UHV set-up is necessary for the whole spectrometer (UHV: UltraHigh Vacuum).

The technical requirements desired according to a) to e) make itnecessary to make specific improvements in certain points on the devicesknown from the above-mentioned state of the art.

The object of the invention is the creation of an apparatus of the kindmentioned at the beginning with which the transport of impurities in aplasma can be measured more accurately than was possible up to now.

The object is achieved by an apparatus having the characterizingfeatures of the main claim. Advantageous embodiments result from thedependent claims.

SUMMARY OF THE INVENTION

The apparatus according to the invention comprises a spectrometer, meansfor the spatially-resolving conversion into charge of light exiting thespectrometer, an integrator circuit for the spatially-resolvingintegration of the charge, as well as means for displaying the chargethat is integrated dependent on the position. The integrator circuit isassembled from discrete components.

The inventor has recognized that the integrator circuit must be modifiedcompared to the state of the art in order to achieve the object of theinvention. In particular, the inventor has recognized that the previouspath, namely to use integrated circuits, must be abandoned, and that theobject can only be achieved by assembling the integrator circuit fromdiscrete components. Discrete components that are suitable for theimplementation arise from the exemplary embodiment.

A large dynamic range at high time resolution (because of the givenvalues for noise level and “full-well-capacity” of the camera head whichis to be connected in series) leads to high total intensities of theamplified spectra at the output, for which an open MCP detector, forexample, then has to be constructed accordingly. By means of a one-stagestandard MCP (with a length/diameter ratio of the individual channels of40:1), a current gain of maximally only about 3000 can be reached. As arule, such a gain is insufficient. A multi-stage standard MCP, as arule, leads to a large spatial broadening (degradation of the spatialresolution and thus the wavelength resolution of the total system).Therefore, the MCP is regularly designed as an EDR-MCP (“Extendeddynamic range”, i.e. a dynamic range extended by the factor 10) with alength/diameter ratio of the individual channels of in particular 60:1(i.e. total gain higher by a factor of 30). It is essential in thisembodiment that the total gain is substantially larger compared to theabove mentioned insufficient gain.

The MCP surface is preferably coated with CsJ in order to increaseefficiency (quantum yield).

For the series-connected phosphor screen for the conversion of theelectron pulse into visible light, a special, very fast phosphor (TypeP46) is chosen in particular, whose afterglow/decay time (some 10 μs) isnoticeably less than the desired time resolution of the total system,and which, in addition, has a large light yield in the wavelength rangein which the series-connected camera head has its highest degree ofefficiency (approx. 500-700 nm). Due to this design, a large gain of theMCP of up to 10⁵ can be achieved in total, at a high degree ofefficiency, high time resolution and acceptable spatial resolution(minimum spot size at the output is approx. 60 μm).

In an embodiment of the invention, the diameter is chosen to be 40 mm(this is the largest available standard diameter) for the MCP and thescreen so that the complete spectrum is shown on it. The spectral linesappear as thin lines on the surface of the phosphor screen which can bemade out with the naked eye (images of the entrance slit).

Essentially, the spatial resolution of the MCP now also determines theoptimum width for the entrance slit up to which the incident amount oflight (and thus the efficiency of the complete system) can be increased,without a noticeable additional deterioration of the wavelengthresolution occurring. Therefore, a width of 50 μm is preferably chosen.

If an additional increase of the incident amount of light is desiredwithout a loss in resolution, the height of the slit is increasedaccording to the invention. This necessitates (because of the 1:1 imagein the spectrometer) a camera head for collecting the spectra whosesensor has as great a height as possible.

Design data on geometry and connection of the camera head. A camerahead/detector is preferred for the collecting the spectra in the exitplane of the MCP detector (phosphor screen) that collects the completespectrum (large sensor width) and collects as much light as possible(large sensor height, perpendicular to the dispersion direction of thespectrometer). Commercially available complete systems that have therequired values of dimensions and time resolution are not existent. Inparticular, no commonly available two-dimensional detectors (e.g. CCDs)with the required measurements are in existence which allow for acomplete readout with an image rate of 1000/s continually. There is,however, a one-dimensional sensor (N-MOS HAMAMATSU Type S3904-1024F withfiber optic entrance window and driver circuit type C4069) with a widthof 25 mm (1024 pixel at 25 μm width each) and a height of 2.5 mm, aswell as a specified pixel rate of up to 2 MHz.

PREFERRED EMBODIMENTS OF THE INVENTION

In one embodiment, a fiber optic cross-section converter (with a ratioof, for example 40/25) is mounted between the (phosphor) screen (widthe.g. approx. 40 mm) and the sensor which makes the image (spectrum)smaller in the exemplary case. In this manner, an adjustment to thegeometrical measurements of the sensor can be carried out. This fiberoptic image is considerably more intense compared to an image created bymeans of lenses, and hence, is to be preferred for this application.

Since the permitted output intensity of the open MCP detector, modulatedup to its linearity limit, does not suffice, as a rule, to modulatesensor S3904 (full capacity 25 pC) sufficiently, in a furtherembodiment, an image intensifier of the first generation (“diode”) witha fiber optic coupling is interposed between the fiber opticcross-section converter and sensor that can bring about an additionallinear light intensification by a factor of 10-15 without significantlosses in spatial resolution.

In the following, an exemplary embodiment of the camera head will bedescribed.

The operation of the used line array S3904-F with the dedicated circuitC 4069 by Hamamatsu requires two kinds of trigger signals that must befed in from the outside: a trigger signal for starting a spectrum mustbe applied at the input “master start”. 6 trigger pulses per pixel mustbe applied to the input “master clock” in order to “push out” the chargeaccumulated on a pixel. Measurement of 1000 spectra per second thereforerequires, in addition to the trigger signal for the spectrum rate (1kHz), a trigger signal of six times the pixel rate, which, at 1024pixels as well as 1000 spectra per second, yields a trigger frequency ofslightly more than 6 MHz. For this task, a commercially availablestandard quartz (6.55 MHz) was selected. The previously describedassembly already permits observation of primitive spectra on anoscilloscope (signal output “data video” or “video out”), for example,when such suitable trigger pulses are applied to the circuit boardC4069. However, the form of the signals of “video out” is given by aseries of short-duration pulses which are hereinafter referred to as“spikes” (half-intensity width approx. 100 ns), and which are verydifficult to collect by means of measurement technology. The physicalraw signal which is the actual signal to be measured is the quantity ofcharge accumulated on one pixel of the line array since the lastmeasurement (previous spectrum). This quantity of charge is proportionalto the area under the spikes at the signal output “video out”. Acorrectly measured collection of these signals therefore requiresintegration of the spikes over time in order to obtain a signal that isproportional to their area. After the measurement of the integratedsignal, this must be deleted in order to reset the circuit before thespike which is to be measured next (switched integrator). Since thespikes are of short duration and the analog signal between two spikescorresponds to zero voltage, an additional analog switch before theinput of the integrator is not necessary.

FIGS. 1 and 2 illustrate the basic structure. The following figuresillustrate the electronics.

In the following, an example for an analog part of the integratingcircuit will be described in detail.

ICs with switched integrators (e.g. Burr-Brown IVC 102) are commerciallyavailable, which, at a time constant (“settling time”) of 6 μs, permitthe integration of a maximum 10⁵ pulses per second; that is too slow byone order of magnitude for an application according to the invention.Therefore, according to the invention, a circuit made of discretecomponents is provided for this task, as can bee seen from the figures.

The centerpiece is a switched (inverting) integrator consisting of afast operational amplifier IC2 (Burr-Brown OPA655, 400 MHz), a resistorR3 (300) and a capacitor C9 (162 pF), with a fast switch IC3 (SiliconixDG 612) being connected in parallel to the capacitor C9 for deleting.

An inverting amplifier IC1 (also OPA 655) is connected before theintegrator for adjusting the level, impedance transformation andinversion of the analog input signals (those are the spikes from thecircuit board C4069, connected to socket B1). When the switch is open,the integrator integrates the applied input voltage according toU_(out)=−(1/RC)∫U_(in)dt+U₀.

The selection of R3 and C9 leads to a time constant RC>>50 ns, which ischosen to be slightly less than the half-intensity width of the spikesso that, only a short time after the peak of the spike, a signal that ispractically constant in time is obtained. However, it must not beselected too small, so that an overdrive of the operational amplifier inthe integrator is avoided.

Deleting the integrated voltage (resetting the capacitor to the initialvalue of U₀=0) is carried out by closing the switch IC3. In the circuitbetween capacitor and switch, there are the resistors R5 and R6 (44 Ohmeach) which, together with the internal resistance of the switch, makesure that the maximum permitted current in the switch IC3 (I_(max)=30mA) is not exceeded during the deleting: The following is valid for theintegrator op-amp: U_(max)=3.5 Volts, and R_(total)=(2*44+45) Ohmsyields I_(max)=26 mA. On the other hand, the time constant of theintegrator circuit is RC=21 ns, so that a far-reaching depletion of thecapacitor to a voltage of less than 0.1% of the initial value can bereached within approx 160 ns, i.e. the next value is independent fromits precursor. The integrated measurement signal is at last output to aBNC socket (B2) via the line driver IC4 (Elantec EL 2003), where thecomponent IC4 makes the connection of a long BNC line (in this case 30m) with 50 Ohms termination possible.

The trigger electronics on the integrator circuit board are built up asfollows.

Elements of the trigger electronics are mounted on the integratorcircuit board that serve the purpose of operating the integrator at theright times (integrating/deleting) and of outputting a trigger signalfor the data recording which is to be done subsequently. A triggersignal (one pulse per pixel), coming from the circuit board C4069, isfed in over the socket B4. This input signal is branched to the ICs 5and 6. At IC5 (Monoflop 74HC221), the pulse operating the integrator isgenerated; the starting time of the deleting pulse (set to 550 ns afterthe peak of the spike) can be set, continually adjustably, by means ofthe potentiometer P1, and the duration of the deleting pulse (set to 160ns) by means of the potentiometer P2, also continually adjustably. Thetrigger pulse for the data recording which is to be done subsequently isgenerated at IC6 (timer component 74HC221); its starting time (set to500 ns) can be set by means of the potentiometer P3 and the duration ofthe pulse (set to 100 ns) can be set by potentiometer P4. This triggerpulse is output to the BNC socket B3 via the driver IC8. Deletion of theintegrated signal therefore takes place shortly (50 ns) after it hasbeen measured, and then, the integrator is again ready for measuring ontime (approx. 710 ns after the peak of the previous signal).

The external triggering will now be explained.

Measuring exactly 1000 complete spectra per second requires feeding atrigger signal for the rate of spectra (“master start”, 1 kHz) as wellan additional trigger signal (“master clock”) consisting of exactly6*1024 pulses which must be applied within a time frame of less than 1ms (else, the pixels that have to be read out already overlap with thenext spectrum). The data sheet for C4069 allows for a master clock ofmaximally 6*2 MHz, however, according to the invention, a frequency of6.55 MHz was selected in order to increase the time between twoconsecutive spikes as much as possible and to simplify the timing forthe operation of the integrator. The two clock frequencies required forC4069 can be generated by means of programmable pulse generators.

In order to ensure a galvanic insulation between the pulse generatorsand the camera head (this is necessary to avoid “ground loops” thatcould lead to disturbances in the measuring signals), the pulses “masterstart” and “master clock” are, at first, led over the BNC sockets B5 andB8, respectively, and fast optical couplers (IC9, IC 10, both OPTOISO1).Furthermore, the “master start” signal is inverted at IC7 (74S140), andthen, both signals are transferred to the circuit board C4069 via thesockets B6 and B7, respectively. These optical couplers are mounted onthe integrator circuit board for reasons of space.

In the following, the voltage supply will be described more closely.

For operation, the integrator circuit board requires highly stabilizedDC voltages +5 V, −5 V, +15 V, −15 V and +6 V that are generated on aseparate circuit board (within the camera head).

A mechanical housing contains the circuit boards C4069, the integratorcircuit board and the voltage supply circuit board. The circuit board ismounted movably and is pressed to the output of the fiber optic coupleron the MCP detector by means of a simple spring mechanism, when thecamera head is mounted to the spectrometer, in order to obtain a goodoptical contact between the two light conductors (entrance window of theline array/fiber optic coupler). From the outside, an alternatingcurrent (two times 18 Volts) is fed into the housing via an insulatedsocket. Furthermore, there are insulated BNC sockets for the triggerinputs (master start and master clock), the trigger output and theintegrated analog signal.

Measures for shielding from electric and magnetic disturbances areprovided as follows.

The occurrence of considerable electromagnetic disturbances must bereckoned with during the camera head's operation in the plasmaexperiment TEXTOR, against which the camera head must be shielded. Thefollowing measures were taken in order to shield the camera head againstthese disturbances: metal housing (aluminum) of the camera head forshielding from electromagnetic fields. A further encasing housing (softiron) for shielding from magnetic fields. All BNC cable joints betweenthe camera head and data recording/triggering are designed as a cablebundle of 30 m length in a joint copper pipe. The signal ground for thecamera head is fed shielded to the camera head by a grounding bar at thedata recording PC. All other connections of the camera head are joinedfree-of-ground or insulated. By means of these measures, it is achievedthat the analog signals of the camera head can be measured with a noiselevel of less than 1 mV, while, at the same time, the position of theline array (correlation of pixel number and wavelength) remains stable.For recording the spectra, a commercial Windows 95 PC is used with adata acquisition card of the Type T112-4 (Co. Imtec, 71522 Backnang) anddata acquisition software INSIGHT (Imtec). Under external triggering (inthis case approx. 1.08 MHz), the system can record the analog data of upto 4 channels simultaneously, continually at a resolution of 12 bit, andbuffer them in the PC's RAM. Given a size of RAM of 256 MB, the data of3 connected camera heads are recorded at TEXTOR continually over a totalmeasurement time of 10 seconds. The analog bandwidth of the card of 500kHz leads to the digitized measurement values recording an average valueover time via the analog signal of the camera head, whereby theaveraging time is approximately 350 ns. This time roughly corresponds tothe time during which the integrated analog signal of the camera headassumes a constant value over time between the end of the spike and thestart of the deleting pulse. The data recording used corresponds to thestate of the art and therefore, is not subject of this patentapplication.

In total, the camera head runs at a continuous rate of spectra of 1000per second with a pixel rate of 1.08 MHz, and, during practicaloperation, reaches a dynamic range for the individual pixel (distancebetween noise and modulation limit) of 10-11 bit. The above-discusseddesign of the individual components leads to the limit of the linearityrange of the MCP being reached just when the IC OPA655 in the integratorcircuit board has reached saturation (approx. 3V). During operation thewith data recording (range of measurement 0 . . . 5 Volts), thiscorresponds to a utilizable linearity range of 2000 counts at approx. 1count noise (i.e. 11 bit utilizable resolution of the complete system).

List of Reference Numerals

1 Diffraction grating

2 Entrance slit

3 Focal plane

4 MCP

5 Screen

6 Fiber optic coupler

7 Image amplifier

8 S3904-F

9 C 4069

10 Integrator circuit board

11 DC voltage source

12 Integrator circuit board

13 Expansion circuit board

14 Inverter

15 Pulse shaper

16 Line driver

17 Analog switch

18 Optical coupler

1. Apparatus comprising: a spectrometer means for thespatially-resolving conversion into charge of light exiting thespectrometer, an integrator circuit for the spatially-resolvingintegration of the charge, means for displaying the charge that isintegrated dependent on the position, characterized in that theintegrator circuit is assembled from discrete components comprising aswitched integrator that consists of an operational amplifier (IC2), Aresistor (R3), and a capacitor (C9) with a switch (IC3) being connectedin parallel to the capacitor C9 for deleting, with resistors (R5 R6)located in the circuit between capacitor (C9) and switch (IC3), which,together with the internal resistance of the switch, make sure that themaximum permitted current in the switch (IC3) is not exceeded during thedeleting.
 2. Apparatus according to claim 1, comprising a detector,namely in particular an MCP detector with a detector surface coated withCsJ.
 3. Apparatus according to claim 1, comprising a phosphor screen forthe conversion of an electron pulse into visible light, with anafterglow/decay time of up to 100 μs, preferably of up to 30 μs. 4.Apparatus according to claim 1, comprising a phosphor screen and asensor with a fiber optic cross-section converter being mounted betweenthem.
 5. Apparatus according to claim 1, comprising a diode which isprovided between the fiber optic cross-section converter and the sensor.